Production of aluminum metal by reduction of aluminum oxide ore is typically carried out in large cells using a molten cryolite electrolyte and carbon block anodes through which massive amounts of electric current are introduced. The carbon blocks form one component of an anode electrode assembly which normally includes a mild steel yoke having a pair of stub ends embedded in the block and coupled to an aluminum stem extending downward from an overhead support. Large numbers of these assemblies may be provided in a single reduction cell.
In operation of such cells, the electrolyte is maintained at a high temperature such as 970.degree. C. and this, in combination with the corrosive nature of the electrolyte, presents a severe environment to the electrode assembly. The carbon block electrode becomes corroded away and must be replaced or refurbished at regular intervals. A large aluminum production facility's electrode refurbishment operations are normally carried out on a continuous basis in a "rodding room" adjacent to the production cell.
Stub ends of the electrode assembly also become corroded away adjacent to their juncture with the carbon block electrode. The stub ends at that location are exposed to fluorine and other fluorine-containing gas or liquid species produced in the molten electrolyte, Na.sub.3 AlF.sub.6, as well as to solid particles of aluminum oxide introduced onto the carbon blocks from above. Stub ends are relatively large in size, for example, six inches in diameter and in practice are made of mild steel for reasons of cost. The operating lifetime of stub ends is limited by their becoming necked-down adjacent to their juncture with the electrode. This reduces the amount of current delivered to the carbon electrode. Prevention of corrosion at this critical location would substantially prolong the operating lifetime of the stubs as well as the assembly as a whole, except for the carbon blocks which are consumed as a part of the normal process.